The FF33 field telephone has a handset (receiver) that contains a microphone (mikrofon) capsule in the mouthpiece. This microphone capsule is powered by a 1.5 volt T30 cell (element). The handset also contains a speaker (fernhörer) in the earpiece. The speaker is an electromagnetic device.
From 1933 until the end of WW2, the German microphone capsules contained carbon powder. When compressed by sound hitting a diaphragm, the carbon powder creates small electrical impulses. These impulses are transmitted over a wire to a connected field telephone. The electromagnet in the handset speaker would convert the impulses into sound waves via a diaphragm.
Unfortunately, the old microphone capsules may work very poorly or stop working altogether. Sometimes the capsules can be brought back to life by tapping them on a hard surface to loosen up the carbon powder. If that fails to work, then you can try a little trick.
Unplug the handset. Using alligator clips, connect wires to Pins 1 and 5. Connect the other ends of the wires to the La and Lb/E terminals on the field telephone. Press in the handset switch while turning the crank on the magneto. It only takes a few turns of the crank. Pull off the wires and plug the handset back in. The microphone should work. If not, the capsule must be replaced.
In the 1950s, the Czech army started using the TP-25 field telephone. The handset of the TP-25 was a copy of the WW2 German FF33, and the microphone capsules even operate on 1.5 volts.
The TP-25 microphone capsule is fully compatible with the FF33. As a matter of fact, the TP-25 capsules seem louder and clearer.
I recently purchased a TP-25 handset for the microphone capsule. Upon installing the capsule in an FF33 handset, I discovered that it was totally dead. I performed the trick described above, and the capsule is now loud and clear.
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WW2 German Field Telephone Equipment 